Ice Ic Without Stacking Disorder by Evacuating Hydrogen from Hydrogen Hydrate

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https://doi.org/10.1038/s41467-020-14346-5 OPEN

Ice Ic without stacking disorder by evacuating hydrogen from hydrogen hydrate

Kazuki Komatsu 1*, Shinichi Machida2, Fumiya Noritake3,4, Takanori Hattori5, Asami Sano-Furukawa5, Ryo Yamane1, Keishiro Yamashita1 & Hiroyuki Kagi1

Water freezes below 0 °C at ambient pressure ordinarily to ice Ih, with hexagonal stacking sequence. Under certain conditions, ice with a cubic stacking sequence can also be formed,

1234567890():,; but ideal ice Ic without stacking-disorder has never been formed until recently. Here we

demonstrate a route to obtain ice Ic without stacking-disorder by degassing hydrogen from

the high-pressure form of hydrogen hydrate, C2, which has a host framework isostructural

with ice Ic. The stacking-disorder free ice Ic is formed from C2 via an intermediate amorphous or nano-crystalline form under decompression, unlike the direct transformations occurring in

ice XVI from neon hydrate, or ice XVII from hydrogen hydrate. The obtained ice Ic shows

remarkable thermal stability, until the phase transition to ice Ih at 250 K, originating from the

lack of dislocations. This discovery of ideal ice Ic will promote understanding of the role of

stacking-disorder on the physical properties of ice as a counter end-member of ice Ih.

1 Geochemical Research Center, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. 2 Neutron Science and Technology Center, CROSS, 162-1 Shirakata, Tokai, Naka, Ibaraki 319-1106, Japan. 3 Graduate Faculty of Interdisciplinary Research, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan. 4 Computational Engineering Applications Unit, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. 5 J- PARC Center, Japan Atomic Energy Agency, 2-4 Shirakata, Tokai, Naka, Ibaraki 319-1195, Japan. *email: [email protected]

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ater freezes below 0 °C at ambient pressure, ordinarily 400 b to ice Ih with a hexagonal stacking sequence. However, VII W ” “ it is also known to produce ice Ic nominally with a C sII 0 C2 cubic stacking sequence under certain conditions1, and its exis- – 300 a c d tence in Earth’s atmosphere2 4, or in comets5,6 is debated. “Ice g C ” fi fi 7 V 1 Ic , or called as cubic ice, was rst identi ed in 1943 by König , lh who used electron microscopy to study the condensation of ice III VI /K 200 VIII from water vapor to a cold substrate. Subsequently, many dif- T II ferent routes to “ice Ic” have been established, such as the dis- sociation of gas hydrates, warming amorphous ices or annealing high-pressure ices recovered at ambient pressure, freezing of μ-or 100 ƒ e nano-confined water (see ref. 1). Despite the numerous studies on “ice Ic”, its structure has not been fully verified, because the dif- “ ” 0 fraction patterns of ice Ic show signatures of stacking 1,8,9 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 disorder , and ideal ice Ic without stacking disorder had not 10 “ ” p/GPa been formed until very recently . This is the reason why ice Ic is double-quoted1, and it is recently proposed that the stacking- 8 disordered ice should not be termed as ice Ic, but as ice Isd . “ ” “Amorphous-like” Ice Ic (ice Isd) is known as a metastable form of ice at atmospheric pressure. But, recent computer simulations suggest state that even ice Isd could be the stable phase for crystallites up to 11 sizes of at least 100,000 molecules . The stability of stacking- Ice I C disordered ices is extremely important because of the ubiquitous c 2 nature of ice. Stacking-disordered ice can be characterized by the Fig. 1 Phase diagram of hydrogen hydrate and ice with experimental degree of ice cubicity, χ, which is defined as the fraction of cubic paths in this study. Phase boundaries for hydrogen hydrates and ices are stacking1,8,9,12,13. Until very recently, the highest cubicity was drawn using thick blue lines and thin black lines, respectively. Experimental 8,14 limited to ~80% , but it has been reported that ideal ice Ic with p-T paths are shown as black arrows in alphabetical sequence from a to g. 10 100% cubicity has been obtained by annealing ice XVII . The structural models for a high-pressure form of hydrogen hydrate, C2, 15 16,17 Recently new ice polymorphs, ice XVI and ice XVII are and ice Ic are schematically drawn with a newly found amorphous-like state obtained by degassing gas molecules from neon and hydrogen as an intermediate transitional state from C2 to ice Ic. Red, white, and light hydrates, respectively. From these findings, we hypothesized that blue balls in the structure model depict oxygen, hydrogen in water ideal ice Ic could be obtained by degassing hydrogen from molecules, and hydrogen in guest molecules, respectively. Note that – hydrogen hydrate, C2. Five different phases in the H2 H2O sys- hydrogens in water molecules are disordered, so that two of four possible tem have been reported to date (see ref. 18): Among them, neu- sites surrounding one oxygen are actually occupied. tron diffraction experiments have never been conducted for the higher-pressure phases, C1 and C2, probably due to the technical difficulty in loading hydrogen into a pressure vessel, or com- 32e site (x, x, x)). The calculated diffraction pattern was in good pressing it to pressures in the giga-pascal range. To synthesize agreement with the observed one, as shown in Fig. 2a. The refined ideal ice Ic, decompression under low-temperature conditions for structural parameters are listed in Supplementary Table 1. degassing is necessary, which is also not straight-forward using The sample was then cooled from 300 K to 100 K at around conventional pressure-temperature controlling systems. We have 3 GPa (path d → e). In the diffraction pattern taken at e in Fig. 1, developed a Mito system19, and have overcome these technical peaks from solid deuterium (phase I) appeared at around 200 K difficulties (see details in Methods). (Supplementary Fig. 2), which is consistent with the known Here we present the neutron and X-ray diffraction results melting curve of hydrogen20. This observation indicates that fluid showing ice Ic without stacking disorder, obtained from degassing deuterium coexisted with C2 through the path from b to d. hydrogen hydrate C2. We also report an unexpected amorphous- The C2 phase persisted at pressures at least as low as 0.5 GPa → like state in the transformation from C2 to ice Ic, and the thermal on decompression at 100 K (path e f). However, surprisingly, stability of ice Ic. the Bragg peaks of C2 mostly disappeared at 0.2 GPa (Fig. 3). This phenomenon is totally unexpected, because the host structure of gas hydrates retains its framework in the previous cases with ice Results XVI15 and XVII16. The sample was further decompressed to The route to obtain ice Ic. We started by using a mixture of D2O 0 GPa and evacuated using a turbo-molecular-pump. The broad and MgD2, which is an internal deuterium source, to synthesize peaks corresponding to ice Ic appeared at this stage. The peak hydrogen hydrate, C2. After loading the mixture into a pressure- disappearance of C2 before the appearance of ice Ic was temperature controlling system, MgD2 was decomposed by reproducibly observed in at least two separate neutron runs and heating at 403 K and at ca. 0 GPa for 1 h through a nominal one X-ray diffraction run for a hydrogenated sample (Supple- + → + + reaction of MgD2 3D2O Mg(OD)2 2D2 D2O (at b in mentary Fig. 3). In the neutron diffraction pattern at 0.2 GPa, Fig. 1, the observed neutron diffraction patterns are shown in except for the Bragg peaks from Mg(OD)2, only a broad peak was Supplementary Fig. 1). Then, the samples were cooled to room observed at around d = 3.75 Å, which was between the peak temperature (at c in Fig. 1) and typically compressed up to positions of 111 of C2 and that of Ice Ic (Fig. 3). This fact implies ~3 GPa until a C2 phase was observed (at d in Fig. 1). that this state does not have long-range periodicity like a normal The neutron diffraction pattern for the C2 phase obtained at crystal, but has only local-ordering like an amorphous or nano- 3.3 GPa and 300 K (at d in Fig. 1) was analyzed by the Rietveld crystal. Considering the observed d-spacing, this amorphous-like method. We adopted a splitting site model for guest D atoms form would be an intermediate transition state from C2 to ice Ic, located at the 48f site (x, 1/8, 1/8), and the host structure was which forms while hydrogen molecules are partially degassed. It is 9 identical to ice Ic (Fd 3m, O at the 8b site (3/8, 3/8, 3/8), D at the highly likely that this apparent amorphization is derived from the

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Ice I a c Iobs Mg(OD)2 C2 Icalc Ice Ic Ih Ibkg Mg(OD)2 Ih I -I * obs calc 250 K Mg(OD)2 240 K 200 K C 2 160 K Intensity/a.u. Ice Ic 0 GPa 130 K 0 GPa 0.2 GPa 0.5 GPa 100 K C2 C2 2.0 2.5 3.0 3.5 4.0 d/Å

b ice Ic Fig. 3 Neutron diffraction patterns showing the transformation from C2 Intensity/a.u. to ice Ic. The patterns were obtained with decreasing pressure at 100 K (path e → f) and with increasing temperature at 0 GPa (path f → g). Corresponding temperatures and pressures are shown at the right side of 101 002 100 the respective patterns, and the arrows mean that temperature or pressure 3.4 3.8 3.6 4.0 kept constant. Most observed peaks are identiﬁed as C2, ice Ic, Mg(OD)2,or ice Ih. The peak marked by an asterisk is a parasitic peak from the high- pressure cell.

Structure refinement for ice Ic. We conducted a separate run in order to obtain a neutron diffraction pattern for the structure fi re nement of the ice Ic. In this run, the neutron diffraction pattern was obtained at 130 K, which is well below the tem- 21 fi perature at which the nucleation of ice Ih occurs . We con rmed that the peak width did not change in the temperature region 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 from 130 K to 180 K, such that the peak sharpening was almost d/Å complete, even at 130 K. The obtained neutron diffraction pattern fi 9 Fig. 2 Results of Rietveld analyses for neutron diffraction patterns. The was well tted using the ice Ic structure model , as shown in Fig. 2b and Supplementary Table 1. We also conducted the patterns of a hydrogen hydrate, C2,andb ice Ic were obtained at 3.3 GPa and 300 K (at d in Fig. 1), and at 0 GPa and 130 K (in the path f → g). The inset Rietveld analysis using C2 structure model, and found that the fl occupancy of the D2 site is zero, within experimental error (occ diffraction pattern in b shows the expanded area for 111 re ections, shown as a = − box in the main figure, with logarithmic scale. The calculated peak positions of (D2) 0.001(1)). This shows that the guest hydrogen molecules are below the detectable limit at 130 K under evacuation. The ice Ih are also shown as light blue lines with their indices in the inset. Structure fi peak pro le around 111 peak of ice Ic has neither the feature of models for C2 and ice Ic are also shown as insets in a and b, respectively. stacking disorder nor the peaks from ice Ih, as shown in the diffraction pattern in the region at around d = 3.9 Å, where the fl lattice mismatch between C2 and ice Ic, originating from the strongest 100 re ection of ice Ih is expected (see inset in Fig. 2b, relatively small cage in the host framework of the ice Ic structure. and more detailed discussion for the peak broadening for ice Ic is From the X-ray diffraction run, ice Ic, which may partially described in Supplementary Note, Supplementary Table 2 and include molecular hydrogen, even appeared at 0.1 GPa through Supplementary Fig. 4). This should be a clear indication of the χ = 13 the transition from the C2 phase to the amorphous-like state, even presence of ideal ice Ic without stacking disorder ( 100%) ,as under pressure (Supplementary Fig. 3). This also represents a clear as the recent discovery of ideal ice Ic by annealing ice difference from the previous cases of ice XVI and XVII; ice XVI is XVII10. formed under evacuation15, and hydrogen molecules can be fi 16 Thermal stability of ice I re lled into ice XVII at an order of 10 bar of pressure .Itis c. It is also noteworthy that the ice Ic worth noting that the partially degassed states are allowed in the surprisingly persists up to at least 240 K until ice Ih started to cases of both ice XVI and XVII, so that the guest molecules can be appear at 250 K (Fig. 3). The temperature of 240 K corresponds to “ ” continuously degassed from a fully occupied state to an empty the upper limit of the reported metastable region of ice Ic (ice 1 state. The observed phase-separation behavior even under Isd) . However, in stacking-disordered ice, the cubic stacking pressure in the ice H2-H2O system indicates that the partially sequence starts to change into a hexagonal stacking sequence at a degassed C2 phase would be unstable compared to the fully much lower temperature, and the phase transition to ice Ih is occupied or emptied phases, probably due to their lattice- completed at 240 K. The notable stability of the ice Ic would be mismatch. derived from the lack of stacking disorder. The stacking- The Bragg peaks in the neutron diffraction pattern for ice Ic disordered ice has more dislocations, which promote the phase obtained at 100 K were still broad, probably due to the small transformation from ice Isd to ice Ih by reducing the activation crystallite size and/or the remaining guest hydrogen molecules. energy required to change the stacking sequence22. This is also The peaks of ice Ic sharpened with increasing temperature. This supported by a recent mesoscopic-size calculation23. Note here sharpening is dependent not only on temperature but also on that the critical temperature of 240 K has been identified as the time, which indicates that it is kinetic behavior. temperature above which ice Ih without cubic stacking faults

NATURE COMMUNICATIONS | (2020) 11:464 | https://doi.org/10.1038/s41467-020-14346-5 | www.nature.com/naturecommunications 3 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-14346-5 forms spontaneously, which is the reason for the anomalous self- cloths were attached on the support rings of the anvils, and the cloths were placed preservation regime of natural gas hydrates24. in contact with the cold head of the cryostat for thermal conduction. The accessible The diffraction pattern observed at 250 K looks a mixture of minimum temperature of the hybrid Mito system is ~35 K, which may be the current technical limitation due to an unavoidable influx of heat from the sur- bulk ice Ic and Ih, rather than stacking-disordered ice with many rounding cell. The sample pressure was estimated from the observed lattice “ ” 29 stacking faults, judging from stackogram reported in the parameter of Mg(OD)2 brucite using the equation of states and the observed unit literatures8,13. At 250 K, crystal growth would be dominant, cell volume of brucite at 0 GPa, assuming the temperature derivative of the bulk rather than crystal nucleation. Therefore, once a crystallite modulus of brucite, dK/dT, was ~0. Although this assumption may cause some error in the pressure estimated at low temperature, we placed emphasis on avoiding nucleates, it quickly grows before other crystallites nucleate, unwanted Bragg peaks from additional sources of pressure marker. Moreover, the resulting in the mixture of ice Ic and Ih, rather than stacking- error would be too small to affect the conclusion. The sample position was aligned disordered ice. This observation also suggests a smaller number of by scanning to maximize the sample scattering intensity. The obtained intensities dislocations in the ice I observed in this study. On the contrary, from the sample in the cell was subtracted by the intensity of the empty cell, and c subsequently normalized by the attenuation corrected intensity of vanadium pellet the remarkable stability of the ice Ic and the bulk mixture of ice Ic in the cell, which was also subtracted by the intensity of the empty cell30. The and Ih at 250 K strongly support the conclusion that the obtained Rietveld analyses were performed using the GSAS31 with EXPGUI32, and the 33 fi ice is not stacking-disordered, and it can therefore be called ice Ic crystal structure was drawn with the VESTA program . The GSAS TOF pro le without the need for quotation marks. function 331 is used as the profile function in the Rietveld analyses. The discovery of ideal ice Ic will allow us to research the real X-ray diffraction. Powder X-ray diffraction measurements using a H O (Milli-Q) physical properties of ice Ic without stacking disorder. For 2 and MgH2 (Wako pure chemical industries, Ltd.) mixture as starting materials example, accurate heat capacity or vapor pressure measurements were performed at the beamline BL-18C in the Photon Factory (KEK, Tsukuba, from low temperature will provide the free energy of ice Ic, which Japan). Samples were exposed to 0.6134 Å monochromatized synchrotron radia- settle the long-standing argument for the thermodynamic tion, and the diffracted scattering was detected by an imaging plate (IP). The pressure was generated using CuBe alloy diamond-anvil cells and the temperature stability of ice Ic compared to ice Ih. The physical properties of the ideal ice I are also important to understand how stacking was controlled using a 4 K GM cryostat (MiniStat, Iwatani Co.) equipped with a c temperature controller (Model 335, Lakeshore). Sample pressure was estimated disorder plays a role in the physical properties of ice Isd. For from the difference in the R1 line wavelengths of rubies inside and outside the fi 34 instance, since the thermal conductivity of ice Isd is signi cantly sample chamber . The temperature was monitored using a Si-diode sensor 25 fi smaller than ice Ih , the difference of thermal conductivities inserted in the cold head edge. We con rmed that the measured temperature was between ices I and I will emboss the effect of stacking disorder. almost the same as that of the diamond anvils after temperature stabilization. The h c experimental p-T path was basically identical to the case of neutron diffraction, as It is also interesting what will be happened when ice Ic is shown in Fig. 1, while the achieved pressure at path d was 4.1 GPa. compressed under low temperature; whichever ice Ic will be One conically shaped Boehler-Almax type diamond-anvil35 with a 0.6 mm culet transformed into HDA (High Density Amorphous ice26) or not? was placed in the direction of the detector with an opening angle of 2θ < 40°, In any case, it is worth conducting what we previously did for ice whereas a conventional anvil with a 0.8 mm culet was positioned in the direction of the X-ray source. A CuBe plate with a hole of diameter 0.3 mm and an initial Ih, to this ideal ice Ic as well. thickness of 0.2 mm was used as a gasket. This gasket was not subjected to pre- indentation. The load was applied by driving the piston by bellows using a He gas Methods cylinder. The bellows allow us to control pressure at a few kbar more precisely than conventionally used membranes. Synthesis of MgD2. MgD2, used as the starting material in this study, was synthesized from reagent-grade MgH2 as follows. MgH2 powder (Wako pure chemical industries, Ltd.) was purchased and further ground in an agate motor to increase DFT calculations. Quantum Espresso36 was used for the DFT calculations37,38.We the surface area, after which it was placed in a copper tube with a diameter of 4 mm used Perdew-Burke-Ernzerhof (so-called PBE) type nonempirical exchange- and a length of 40 mm. The tube was mechanically sealed and but not welded, correlation functions39 for this study. The pseudopotentials were derived using allowing the transfer of hydrogen gas. The copper tube was inserted into a 1/4” projector augmented-wave approximation40. The dispersion effects were taken into Inconel tube and connected in parallel to a deuterium gas cylinder and a turbo- account using the exchange-hole dipole moment method (XDM), which calculates molecular pump (TMP) with 1/16” stainless tubes and stop bulbs. The Inconel coefficients for polynomial of DFT-D dispersion energy41 from the exchange-hole tube, including the sample copper tube, was heated to 773 K for 1 h using a tube dipole moment calculated from simulated electron wave function42,43. XDM 44 furnace under evacuation using the TMP. Under these conditions, MgH2 com- damping function parameters are taken from . The enthalpies of four possible 27 fi pletely decomposed to Mg and H2 , and the degassed H2 was evacuated. Then, the con gurations for the ordered form of ice Ic were calculated within a unit cell with D2 gas was introduced up to 4 MPa, and the temperature was cycled at the rate of a kinetic energy cutoff of 70 Ry and a Brillouin zone k mesh of 8 × 8 × 8. The 1 K/min between 673 K and 773 K while keeping the pressure at 4 MPa, which cell parameters and atomic coordinates were optimized using BFGS quasi- 27 represents stable and unstable conditions for MgH2 , and this temperature cycle Newtonian methods at atmospheric pressure. was repeated for 20 times. This activation process is necessary for the reaction + → Mg D2 MgD2. Finally, the p-T conditions were maintained at 673 K and 4 MPa for 3 days to complete the reaction. The recovered sample was analyzed by Data availability fi The primary data that support the plots within this paper and other finding of this study powder X-ray diffraction (MiniFlex-II, Rigaku) and identi ed to be MgD2 with a trace amount of MgO. Both MgD2 and MgO react with D2O and produce Mg are available from the corresponding author on reasonable request. The neutron (OD)2, so this small amount of MgO does not affect the conclusion of this study. crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 1973757, 1973759. These data can be obtained free of charge from The Cambridge Neutron diffraction and p-T control . Neutron powder diffraction experiments Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. were conducted at the beamline PLANET28 in the Material and Life Science Experiment Facility (MLF) of J-PARC, Ibaraki, Japan. The incident beam consists of 25 Hz pulsed spallation neutrons produced from a liquid Hg target via a Received: 23 September 2019; Accepted: 3 January 2020; decoupled moderator and traveled through collimators, choppers and supermirror guides to the sample positioned at 25.0 m from the moderator28. Approx. 20 mg of fi MgD2, synthesized as described above, was lled into TiZr null scattering gaskets, and D2O water (99.9%, Wako pure chemical industries, Ltd.) was dropped on the MgD2 powder, resulting in the molar ratio of MgD2:D2O~1:3. The gaskets were sandwiched between a pair of tungsten carbide anvils, and loaded by using a hybrid Mito system, which is a modified version of an original pressure-temperature References 19 fl 1. Kuhs, W. F., Sippel, C., Falenty, A. & Hansen, T. C. Extent and relevance of variable Mito system . The hybrid Mito system uses both owing liquid nitrogen “ ” 109 – and a 4 K cryostat (RDK-415D, Sumitomo Heavy Industries, Ltd.), which allows us stacking disorder in ice Ic . Proc. Natl Acad. Sci. USA , 21259 21264 (2012). to control temperature rapidly, owing to the large latent heat of liquid nitrogen and 2. Murphy, D. M. Dehydration in cold clouds is enhanced by a transition from 30 efficient thermal insulation by zirconia and GFRP seats. 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Crystal structure of magnesium dichloride decahydrate Open Access This article is licensed under a Creative Commons determined by X-ray and neutron diffraction under high pressure. Acta Attribution 4.0 International License, which permits use, sharing, Crystallogr. B 71,74–80 (2015). adaptation, distribution and reproduction in any medium or format, as long as you give 31. Larson, A. & Von Dreele, R. General Structure Analysis System (GSAS). Los appropriate credit to the original author(s) and the source, provide a link to the Creative Alamos National Laboratory, Report LAUR-86-748 (2004). Commons license, and indicate if changes were made. The images or other third party 32. Toby, B. EXPGUI, a graphical user interface for GSAS. J. Appl. Crystallogr. 34, material in this article are included in the article’s Creative Commons license, unless 210–213 (2001). indicated otherwise in a credit line to the material. If material is not included in the 33. Momma, K. & Izumi, F. VESTA 3 for three-dimensional visualization of crystal, article’s Creative Commons license and your intended use is not permitted by statutory volumetric and morphology data. J. Appl. Crystallogr. 44, 1272–1276 (2011). regulation or exceeds the permitted use, you will need to obtain permission directly from 34. Piermarini, G. J., Block, S., Barnett, J. D. & Forman, R. A. Calibration of the the copyright holder. To view a copy of this license, visit http://creativecommons.org/ fl pressure dependence of the R1 ruby uorescence line to 195 kbar. J. Appl. licenses/by/4.0/. Phys. 46, 2774–2780 (1975). 35. Boehler, R. & De Hantsetters, K. New anvil designs in diamond-cells. High. Press. Res. 24, 391–396 (2004). © The Author(s) 2020

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